Convection
Convection is the movement of molecules within fluids (i.e. liquids, gases) and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids.
Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.[1]
Note that a common use of the term convection refers specifically to heat transfer by convection, as opposed to convection in general.
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Terminology
The term "convection" may have slightly different but related usages in different contexts. The broader sense is in fluid mechanics, where "convection" refers to the motion of fluid (regardless of cause)[2]. However in thermodynamics "convection" often refers specifically to heat transfer by convection[3].
Additionally, convection includes fluid movement both by bulk motion (advection) and by the motion of individual particles (diffusion). However in some cases, convection is taken to mean only advective phenomena. For instance, in the transport equation, which describes a number of different transport phenomena, terms are separated into "convective" and "diffusive" effects. A similar differentiation is made in the Navier–Stokes equations. In such cases the precise meaning of the term may be clear only from context.
Examples and applications of convection
Convection occurs on a large scale in atmospheres, oceans, and planetary mantles. Fluid movement during convection may be invisibly slow, or it may be obvious and rapid, as in a hurricane. On astronomical scales, convection of gas and dust is thought to occur in the accretion disks of black holes, at speeds which may closely approach that of light.
Heat transfer
Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion (observable movement) of fluids. Heat is the entity of interest being advected (carried), and diffused (dispersed). This can be contrasted with conductive heat transfer, which is the transfer of energy by vibrations at a molecular level through a solid or fluid, and radiative heat transfer, the transfer of energy through electromagnetic waves.
Heat is transferred by convection in numerous examples of naturally ocurring fluid flow, such as: wind, oceanic currents, and movements within the earth's mantle. Convection is also used in engineering practices to provide desired temperature changes, as in heating of homes, industrial processes, cooling of equipment, etc.
The rate of convective heat transfer may be improved by the use of a heat sink, often in conjuction with a fan. For instance, a typical computer CPU will have a purpose-made fan to ensure its operating temperature is kept within tolerable limits.
Convection cells
A convection cell, also known as a Bénard cell is a characteristic fluid flow pattern in many convection systems. A rising body of fluid typically loses heat because it encounters a cold surface; because it exchanges heat with colder liquid through direct exchange; or in the example of the Earth's atmosphere, because it radiates heat. Because of this heat loss the fluid becomes denser than the fluid underneath it, which is still rising. Since it cannot descend through the rising fluid, it moves to one side. At some distance, its downward force overcomes the rising force beneath it, and the fluid begins to descend. As it descends, it warms again and the cycle repeats itself.
Atmospheric circulation
Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller ocean circulation) by which thermal energy is distributed on the surface of the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains fairly constant.
Latitudinal circulation is the consequence of the fact that incident solar radiation per unit area is highest at the heat equator, and decreases as the latitude increases, reaching its minimum at the poles. It consists of two primary convection cells, the Hadley cell and the polar vortex.
Longitudinal circulation, on the other hand, comes about because water has a higher specific heat capacity than land and thereby absorbs and releases more heat, but the temperature changes less than land. This effect is noticeable; it is what brings the sea breeze, air cooled by the water, ashore in the day, and carries the land breeze, air cooled by contact with the ground, out to sea during the night. Longitudinal circulation consists of two cells, the Walker circulation and El Niño / Southern Oscillation.
Weather
More localized phenomena than global atmospheric movement are also due to convection, including wind and some of the hydrologic cycle. For example, a foehn wind is a type of down-slope wind which occurs in the downwind side of a mountain range. It results from the adiabatic warming of air which has dropped most of its moisture on windward slopes. As a consequence of the different adiabatic lapse rates of moist and dry air, the air on the leeward slopes becomes warmer than equivalent elevations on the windward slopes, leading to the wind.
A thermal column (or thermal) is a vertical section of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation. The Sun warms the ground, which in turn warms the air directly above it. The warmer air expands, becoming less dense than the surrounding air mass. The mass of lighter air rises, and as it does, it cools due to its expansion at lower high-altitude pressures. It stops rising when it has cooled to the same temperature as the surrounding air. Associated with a thermal is a downward flow surrounding the thermal column. The downward moving exterior is caused by colder air being displaced at the top of the thermal.
Another convection-driven weather effect is the sea breeze.
Oceanic circulation
Solar radiation affects the oceans: warm water from the Equator tends to circulate toward the poles, while cold polar water heads towards the Equator. Oceanic convection is also frequently driven by density differences due to varying salinity, known as thermohaline convection, and is of crucial importance in global ocean circulation. In this case it is possible for relatively warm, saline water to sink, and colder, fresher water to rise, reversing the normal transport of heat.
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the earth to the surface. It is the driving force that causes tectonic plates to move around the Earth's surface.
The Earth's surface is divided into a number of tectonic plates that are continuously being created and consumed at their opposite plate boundaries. Creation (accretion) occurs as mantle is added to the growing edges of a plate. This hot added material cools down by conduction and convection of heat. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction at an ocean trench. This subducted material sinks to some depth in the Earth's interior where it is prohibited from sinking further. The subducted oceanic crust triggers volcanism.
Stack effect
The Stack effect or chimney effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers due to buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect helps drive natural ventilation and infiltration. Some cooling towers operate on this principle; similarly the solar updraft tower is a proposed device to generate electricity based on the stack effect.
Stellar physics
The convection zone of a star is the range of radii in which energy is transported primarily by convection.
Granules on the photosphere of the Sun are convection cells caused by convection of plasma. The rising part of the granules is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. A typical granule has a diameter on the order of 1,000 kilometers and lasts 8 to 20 minutes before dissipating. Below the photosphere is a layer of "supergranules" up to 30,000 kilometers in diameter with lifespans of up to 24 hours.
The image shows the solar photosphere where granules are visible. North America is superimposed to provide a sense of scale.
Convection mechanisms
Convection may happen in fluids at all scales larger than a few atoms. There are a variety of circumstances in which the forces required for natural and forced convection arise, leading to different types of convection, described below. In broad terms, convection arises because of body forces acting within the fluid, such as gravity (buoyancy), or surface forces acting at a boundary of the fluid.
The causes of convection are generally described as one of either "natural" ("free") or "forced", although other mechanisms also exist (disscussed below). However the distinction between natural and forced convection is particularly important for convective heat transfer.
Natural convection
Natural convection, or free convection, occurs due to temperature differences which affect the density, and thus relative buoyancy, of the fluid. Heavier (more dense) components will fall while lighter (less dense) components rise, leading to bulk fluid movement. Natural convection can only occur, therefore, in a gravitational field. A common example of natural convection is a pot of boiling water in which the hot and less-dense water on the bottom layer moves upwards in plumes, and the cool and more dense water near the top of the pot likewise sinks.
Natural convection will be more likely and/or more rapid with a greater variation in density between the two fluids, a larger acceleration due to gravity that drives the convection, and/or a larger distance through the convecting medium. Convection will be less likely and/or less rapid with more rapid diffusion (thereby diffusing away the gradient that is causing the convection) and/or a more viscous (sticky) fluid.
The onset of natural convection can be determined by the Rayleigh number (Ra).
Note that differences in buoyancy within a fluid can arise for reasons other than temperature variations, in which case the fluid motion is called gravitational convection (see below).
Forced convection
In forced convection, also called heat advection, fluid movement results from external surface forces such as a fan or pump. Forced convection is typically used to increase the rate of heat exchange. Many types of mixing also utilize forced convection to distribute one substance within another. Forced convection also occurs as a by-product to other processes, such as the action of a propeller in a fluid or aerodynamic heating. Fluid radiator systems, and also heating and cooling of parts of the body by blood circulation, are other familiar examples of forced convection.
Forced convection may produce results more quickly than free convection. For instance, a convection oven works by forced convection, as a fan which rapidly circulates hot air forces heat into food faster than would naturally happen due to simple heating without the fan.
Gravitational or buoyant convection
Gravitational convection is a type of natural convection induced by buoyancy variations resulting from material properties other than temperature. Typically this is caused by a variable composition of the fluid. If the varying property is a concentration gradient, it is known as solutal convection[4]. For example, gravitational convection can be seen in the diffusion of a source of dry salt downward into wet soil due to the buoyancy of fresh water in saline. [5]
Variable salinity in water and variable water content in air masses are frequent causes of convection in the oceans and atmosphere which do not involve heat, or else involve additional compositional density factors other than the density changes from thermal expansion (see thermohaline circulation). Similarly, variable composition within the Earth's interior which has not yet achieved maximal stability and minimal energy (in other words, with densest parts deepest) continues to cause a fraction of the convection of fluid rock and molten metal within the Earth's interior (see below).
As buoyant convection is due to the effects of gravity, it does not occur in microgravity environments.
Granular convection
Vibration-induced convection occurs in powders and granulated materials in containers subject to vibration where an axis of vibration is parallel to the force of gravity. When the container accelerates upward, the bottom of the container pushes the entire contents upward. In contrast, when the container accelerates downward, the sides of the container push the adjacent material downward by friction, but the material more remote from the sides is less affected. The net result is a slow circulation of particles downward at the sides, and upward in the middle.
If the container contains particles of different sizes, the downward-moving region at the sides is often narrower than the largest particles. Thus, larger particles tend to become sorted to the top of such a mixture. This is one possible explanation of the Brazil nut effect.
Thermomagnetic convection
Thermomagnetic convection can occur when an external magnetic field is imposed on a ferrofluid with varying magnetic susceptibility. In the presence of a a temperature gradient this results in a nonuniform magnetic body force, which leads to fluid movement. A ferrofluid is a liquid which becomes strongly magnetized in the presence of a magnetic field.
This form of heat transfer can be useful for cases where conventional convection fails to provide adequate heat transfer, e.g., in miniature microscale devices or under reduced gravity conditions.
Capillary action
Capillary action is a phenomenon where liquid spontaneously rises in a narrow space such as a thin tube, or in porous materials. This effect can cause liquids to flow against the force of gravity. It occurs because of inter-molecular attractive forces between the liquid and solid surrounding surfaces; If the diameter of the tube is sufficiently small, then the combination of surface tension and forces of adhesion between the liquid and container act to lift the liquid.
Marangoni effect
The Marangoni effect is the convection of fluid along an interface between dissimilar substances because of variations in surface tension. Surface tension can vary because of inhomogeneous composition of the substances, and/or the temperature-dependence of surface tension forces. In the latter case the effect is knowns as thermo-capillary convection.
A well-known phenomenon exhibiting this type of convection is the "tears of wine".
Weissenberg effect
The Weissenberg effect is a phenomenon that occurs when a spinning rod is placed into a solution of liquid polymer. Instead of being thrown outward, entanglements cause the polymer chains to be drawn towards the rod instead of being thrown outward as would happen with an ordinary fluid (i.e., water).
Combustion
In a zero-gravity environment, there can be no buoyancy forces, and thus no natural (free) convection possible, so flames in many circumstances without gravity, smother in their own waste gases. However, flames may be maintained with any type of forced convection (breeze); or (in high oxygen environments in "still" gas environments) entirely from the minimal forced convection that occurs as heat-induced expansion (not buoyancy) of gases allows for ventilation of the flame, as waste gases move outward and cool, and fresh high-oxygen gas moves in to take up the low pressure zones created when flame-exhaust water condenses.[6]
Mathematical models of convection
Mathematically, convection can be described by the convection–diffusion equation or the generic scalar transport equation.
Quantifying natural versus forced convection
In cases of mixed convection (natural and free ocurrnig together) one would often like to know how much of the convection is due to external constraints, such as the fluid velocity in the pump, and how much is due to natural convection occurring in the system.
The relative magnitudes of the Grashof and Reynolds number squared determine which form of convection dominates. if 
forced convection may be neglected, whereas if 
natural convection may be neglected. If the ratio is approximately one both forced and natural convection need to be taken into account.
See also
- Atmospheric convection
- Bénard cells
- Churchill-Bernstein Equation
- Double diffusive convection
- Fluid dynamics
- Heat transfer
- Heat pipe
- Laser-heated pedestal growth
- Nusselt number
- Thermomagnetic convection
- Vortex tube
References
- ↑ Frank P. Incropera; David P. De Witt (1990). Fundamentals of Heat and Mass Transfer (3rd ed.). John Wiley & Sons. ISBN 0-471-51729-1.
- ↑ Fundamentals of Fluid Mechanics, by Bruce R. Munson, ISBN 13: 9780471855262, ISBN 10: 047185526X, John Wiley & Sons Inc
- ↑ Thermodynamics:An Engineering Approach, by Yunus A. Çengel, Michael A. Boles, ISBN 007121688X / 9780071216883 / 0-07-121688-X, McGraw-Hill Education
- ↑ http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.15.8288
- ↑ Steady Gravitational Convection Induced by a Line Source of Salt in a Soil - Raats 33 (4): 483 - Soil Science Society of America Journal
- ↑ Does a candle burn in zero-g?
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